1. Field of the Invention
The present invention relates to a power-factor improvement converter for improving the power-factor of a switching power circuit.
2. Description of Related Art
Owing to the recent development and advancement of switching elements that can withstand a comparatively high current and voltage of high frequency, the switching power supply has become very popular in obtaining a desired dc voltage by rectifying the power-frequency power source.
The switching power supply can be made smaller because transformers and other components can be made smaller by setting the switching frequency higher, so that it has been widely applied to the power supply of various electronic equipment as a high-power DC-DC converter.
Generally, rectifying the power-frequency power source produces a distorted waveform running through a smoothing circuit, which lowers the power-factor that indicates the efficiency in use of the power.
And, the distorted current waveform creates interfering harmonics, thereby requiring a countermeasure to suppress the harmonics.
As means for improving the power-factor in the switching power circuit, a method of using the so-called active filter is known which applies a boosting converter by the PWM control system to a rectifying circuit to increase the power-factor to 1.
FIG. 7 is a circuit diagram showing an example of a switching power circuit that the present inventor has proposed aiming at improving the power-factor using the foregoing active filter.
In the power circuit shown in FIG. 7, a common mode choke coil CMC and across condenser C.sub.L are used as a noise filter for removing common mode noises to the AC power-frequency.
The AC power-frequency is rectified in full-wave by a bridge rectifying circuit D.sub.1. In this case, an active filter 20 is interposed between the output of the bridge rectifying circuit D.sub.1 and a smoothing condenser Ci of a smoothing circuit, and it is to improve the power-factor as described later.
The switching power circuit 1 is a DC-DC converter, receiving an input voltage E.sub.1 that is rectified and smoothed at both ends of the smoothing condenser Ci and performing a switching operation to supply dc output voltage E.sub.1, E.sub.2. In this case, the foregoing switching power circuit 1 is equipped with a switching converter for controlling to stabilize the output by the PWM system. Further, the active filter is a boosting type, the dc voltage (rectified and smoothed voltage E.sub.1) generated by this active filter 20 is controlled to a constant, for example, about 380 V against the variation of the AC input voltage.
Next, the construction of the active filter 20 will be described.
In the active filter 20 shown in FIG. 7, a filter choke coil L.sub.N --winding Li of choke coil CH--ferrite beads FB.sub.2 --high-speed recovery type rectifying diode D.sub.12 are connected in series.
Two filter condensers C.sub.N are connected from both ends of the filter choke coil L.sub.N to the primary ground, and the filter choke coil L.sub.N and the filter condensers C.sub.N, C.sub.N form the so-called .pi.-type normal mode low-pass filter. And, the aforementioned common mode and the normal mode noise filter prevent harmonic noises including switching noises from flowing into the AC power-frequency power source.
The winding Li of choke coil CH is interposed as an energy storage means being a higher voltage source or higher current source than the rectified voltage, so that it can pour current into the load (switching converter) during the open period of a switching element Q.sub.20 described later.
And, the high-speed recovery type rectifying diode D.sub.12 is interposed corresponding to a high frequency current running out into the rectifier output accompanying with the switching operation of the switching element Q.sub.20.
The rectified current running into the rectifier output through the winding Li of choke coil CH and the high-speed recovery type rectifying diode D.sub.12 is charged into the smoothing condenser Ci, at both ends of which the rectified and smoothed voltage is obtained which is served as the operating power source for the following switching power circuit 1.
And, this case applies, for example, a MOS-FET transistor to the switching element Q.sub.20 forming the active filter. The drain of the FET is connected to the winding Li of choke coil CH via ferrite beads FB.sub.1 and to the anode of the high-speed recovery type rectifying diode D.sub.12 through the ferrite beads FB.sub.1 and FB.sub.2. The source of the FET is connected to the primary earth via a rush current limiting resister R.sub.D1. The switching element Q.sub.20 receives at the gate a switching drive signal from a drive circuit in the active filter control circuit 20 described later, and thereby it performs a switching operation.
And, in this active filter 20, a snubber circuit composed of the ferrite beads FB.sub.1 and a condenser C.sub.S1 and resister R.sub.5A is provided for the foregoing switching element Q.sub.20, and another snubber circuit composed of the ferrite beads FB.sub.2 and a condenser C.sub.S2 and resister R.sub.5B is provided for the high-speed recovery type rectifying diode D.sub.12.
The switching element Q.sub.20 and the high-speed recovery type rectifying diode D.sub.12 each perform the switching operation based on the PWM control of the active filter control circuit 20A, as described below. Since the rise/fall time of turn-on/turn-off current thereat is short, the switching operation involves radiation noises of comparably high levels. Therefore, interposing the foregoing snubber circuits blunts the slope of the rise/fall of the switching current waveform, thereby suppressing the radiation noises.
The active filter control circuit 20A controls the operation of the active filter that improves the power-factor so as to raise to 1, which is composed of, for example, one integrated circuit (IC).
The active filter control circuit 20A comprises a starting circuit for driving the switching element Q.sub.20 when the power is turned on, an oscillating circuit for generating a specific switching frequency, a drive circuit for amplifying the signal of the foregoing oscillating frequency and generating a gate signal to drive the switching element Q.sub.20, a PWM control circuit for controlling a switching drive signal fed from the foregoing drive circuit by the PWM control system, and a multiplier that multiplies on the basis of the input of a feed-forward and feedback circuit described below to generate a control input signal for the foregoing PWM control circuit.
A dividing resister R.sub.1 and R.sub.2 in series are interposed in parallel between the positive output terminal of the bridge rectifying circuit D.sub.1 and the primary ground. The divided potential by the dividing resister R.sub.1 and R.sub.2 is entered into the active filter control circuit 20A, which forms the feed-forward circuit corresponding to the AC input voltage.
And, the feedback circuit is formed such that a dividing resister R.sub.3 and R.sub.4 divide the both end voltage (rectified smoothed voltage) across the smoothing condenser Ci and the divided potential is entered into the active filter control circuit 20A. Namely, the active filter control circuit 20A receives a voltage corresponding to the AC input voltage through the feed-forward circuit and a voltage corresponding to the rectified smoothed voltage through the feedback circuit.
Further, the output of a half-wave rectifying circuit by a winding N.sub.5 wounded on the choke coil CH and a rectifying diode D.sub.6 is supplied to the active filter control circuit 20A as the operating power source.
The operation will now be outlined on how the power-factor is improved by the aforementioned active filter.
The active filter control circuit 20A detects an AC input voltage on the basis of the voltage entered through the feed-forward circuit and enters it into the multiplier inside. On the other hand, the active filter control circuit 20A detects a varying difference of the rectified smoothed voltage on the basis of the voltage entered through the feedback circuit. The active filter control circuit 20A controls the average voltage of the rectified smoothed voltage Ei to stabilize within 360 V-380 V on the basis of the varying difference of the rectified smoothed voltage and enters the varying difference of the rectified smoothed voltage into the multiplier inside.
And, the multiplier multiplies the AC input voltage and the varying difference of the rectified smoothed voltage which are detected as described above. The multiplication creates a command value of current with the same waveform as the AC input voltage V.sub.AC.
And, the PWM control circuit compares the foregoing current command value with the actual AC input current, and creates a PWM signal corresponding to the difference to supply it to the drive circuit. The switching element Q.sub.20 is driven by a drive signal based on the PWM signal. Consequently, the AC input current is controlled so as to have the identical waveform to the AC input voltage, whereby the power-factor is improved to come close to almost 1. In this case, the power-factor of 0.95-0.99 can be obtained against the AC input voltage and load variation.
And also in this case, since the current command value created by the multiplier is controlled so that the amplitude can vary corresponding to the varying difference of the rectified smoothed voltage, the variation of the rectified smoothed voltage is also suppressed.
In the power circuit shown in FIG. 7, the active filter control circuit 20 is used as a the power-factor improvement converter, and the components are expensive as well as the number of the components is comparably large, which is disadvantageous in producing the circuit in a small size and at a low cost.
And, in the power circuit shown in FIG. 7, since the active filter control circuit 20 and the following switching power circuit 1 perform the switching operation according to the PWM control system, the operation handles rectangular waveform, which inevitably involves a high radiation of EMI (electromagnetic interference). To take a measure for this, it is necessary, for example, to enhance the performance of the normal mode low-pass filter (L.sub.N and C.sub.N, C.sub.N) and the common mode noise filter (CMC, C.sub.L), thus it leads to a big size of components for the noise filter and increasing the production cost.
And, as a measure for the EMI, applying the foregoing snubber circuit to the switching element Q.sub.20 and the high-speed recovery type rectifying diode D.sub.12 in the active filter control circuit 20 is known to increase a power loss for the insertion; for example, in the active filter control circuit 20 in FIG. 7, the AC-DC power conversion efficiency reduces to about 90%. Therefore, assuming that the DC-DC conversion efficiency of the switching power circuit 1 is about 85%, the overall power conversion efficiency as the total power circuit is reduced to 90%.times.85%=76.5%.
Furthermore, in the active filter control circuit 20 shown in FIG. 7, since the dc output voltage (rectified smoothed voltage Ei) is controlled to be about 380 V constant, applying an existing switching power circuit for 100 VAC to the 100 VAC system is impossible as it is. various design modifications including increase of withstand voltage of the switching element are needed, which is also disadvantageous in regard to cost.